Digimodes on 11 meters

darkmaster2

Anyone here interested in digimodes on 11 meters?

SSTV
ROS
PSK?

darkmaster2

Does anyone know if there are restrictions on using digimodes on the CB radio band using legal power limits and radios?

watty

Theoretically you can't, in practice if you keep power down (you can't run digimodes at full power without REDUCING range and creating interference due to distortion) and don't annoy anyone it's unlikely you'll have a problem.

Even on quality Amateur rigs (and there are few quality CB rigs) you have to run about 1/4 power max for best results unless it's something like RTTY. You need as little distortion as possible to get the maximum range.

SSTV an issue is "duty cycle" of tx. An image takes quite a while and has a constant power density, so while not so prone to distortion as PSK, your rig needs good heatsink. It can be run more nearly full power.

You need SSB mode and audio in & TX without any AVC/AGC/ALC. Set rig to full power and then back it off to lower power with microphone knob. Note Laptop sound card settings and mic gain for each mode. You need an adaptor from computer sound out to mic socket (optionally a transistor from serial port handshake to Mic PTT switch contact) and from Ext socket on CB to computer sound in.

You can't modify the CB as that invalidates its legal status.

It's explicitly forbidden on PMR446

darkmaster2

Cheers for that info watty,
27.245 is very active at the moment with ROS, RTTY, PSK and various other modes, mostly from germany,netherland, France, and spain ever since ssb became legal. It would be great to get some sort of band plan like they have on the Ham bands, assign one or two channels to digimodes and therefore eliminate the qrm from am/fm stations but then again who would enforce it.

Hopefully some decent quality legal ssb cb radios will come out soon. I must have a look around and see whats about.

brownmini

darkmaster2 wrote: »

Does anyone know if there are restrictions on using digimodes on the CB radio band using legal power limits and radios?

Based upon previous publications from COMREG that mention
various ETSI standards etc, I have bad news.

Both 300-135 and 300-433 of the ETSI standards mention voice and/or
data transmissions.

In the exemption SI ( S.I. No. 436 of 1998.) specifically states that the
exemption from requiring a licence only applies to the transmitting and
receiving of *SPOKEN* messages.

You can get the above mentioned SI from the following link...
http://www.comreg.ie/_fileupload/publications/SI436of1998.pdf
and the VOICE ONLY bit is in section 4 just above ''Given under my hand..."

73

watty

Yes

watty wrote: »

Theoretically you can't

Is 27.245 actually *IN* the licence free CB band or UK only?

brownmini

watty wrote: »

Yes



Is 27.245 actually *IN* the licence free CB band or UK only?

There's no such thing as ' free'

And I suggest that to remain licence free, the CB has to obey the ETSI
paperwork and the ETSI paperwork clearly lists the channels and the
frequencies associated with each channel but at least one of the ETSI's
(cant remember which one) does give a swing of 10kHz if you read it one
way which might be handy for your question to get a yes answer
[Long shot]

darkmaster2

Thanks for the info.
Is there anywhere I can find the regulations for each European country?

The only country I know that of that allows digimodes on CB is Germany , on the specific frequencies 26.565, 26.675, 26.685, 26.915, 26.925, 27.025, 27.035, 27.235, 27.245, and 27.405 MHz. When conditions are good you can hear a lot of packet radio,psk and ROS activity.

On the international 11m 'free band' , Pirate band or whatever you want to call it:
27.700usb is usually busy with sstv analogue, 27.720usb is used for easypal, and 27.500usb is used for psk (none legal in any country)

The 40 channel legal CB band is almost completely dead in Ireland anyway. Is it worth contacting comreg to allow packet or other digmodes on a couple of legal cb channels citing Germany as an example.? It might bring some life to an otherwise dead hobby in Ireland.

watty

Not unless there is vast revenue for the Government and profits for Mobile Operators. That is all Comreg cares about.

The RF Tech Guy

watty wrote: »

Theoretically you can't, in practice if you keep power down (you can't run digimodes at full power without REDUCING range and creating interference due to distortion) and don't annoy anyone it's unlikely you'll have a problem.

If you run a road vehicle that is not exempt from using marked diesel fuel it is breaking the law. Theoretically you can but if you are caught you will face a fine etc... Legislation does not currently allow for digimodes on CB in Ireland. Poor advice from a communications engineer who engages Comreg on regulatory issues....:eek:

watty wrote: »

Even on quality Amateur rigs (and there are few quality CB rigs) you have to run about 1/4 power max for best results unless it's something like RTTY. You need as little distortion as possible to get the maximum range.

Incorrect. It is quite possible to run full power on an amateur radio transmitter for digimodes. You can ensure you transmitter is as linear as possible and use active linearisation.

watty wrote: »

It's explicitly forbidden on PMR446

How is this relevant to the discussion? Its explicitly forbidden to burn your household rubbish outdoors, but that piece of information is in no way relevant to the discussion either.....

martinedwards

the reason folks run lower power on SSTV is that its a constant signal and it puts far more strain on the output section than SSB which flickersw all over the place (outputwise) and so doesn't cook the rig.

my humble opinion?

CB is designed for non technically minded folk to have 2 way radio.

if you want to get into this kind of stuff, then you are obviously bright enough to do the test and become a Ham where there will be others who will not only be interested in your signal, but not swear violently at you for jamming thier chat about the weather in Clones.

watty

It is quite possible to run full power on an amateur radio transmitter for digimodes.

Yes, but no commercial Ham radio is linear enough. You get MORE range backing off the power on PSK etc. No Commercial Digital transmitter is run at the FM or AM power it can do for this reason. ANY distortion reduces the "hamming distance" between symbols, effectively degrading the BER (Digital Signal to Noise).

Pure FM / CW / Dual tone FSK can run at full power (Though not for too long on many sets. Look at difference in cooling FT857 and FT897, both "nominally" 100W CW/SSB/FM, 25W AM
"real" Digital mode will be worse above a certain power level.

The RF Tech Guy

BER = Bit Error Ratio, not digital signal to noise.

Try an Elecraft K3 with active linearisation set up properly it is possible to produce a very "clean" signal

watty

How do think "Digital" as opposed to to Analogue "noise" is usefully specifed? BER is what counts.

Yes a K3 is good but not able to run QPSK at the same power as FM. That's fantasy. Nor is it "typical" either.

The RF Tech Guy

Noise by its nature is analogue.

When dealing with digital modes, it is Carrier to Noise ratio that should be referred to rather than Signal to Noise ratio.

To acheive a good BER for a particular modulation scheme, a minimum Carrier to Noise ratio will be required.

Anyhow, this is wandering off topic.

watty

Read up on what distortion does to the allowable noise before you can't tell which symbol it is reliably.

Read about "hamming distance"

The more distortion the less noise the signal can endure without more errors.

(Simplistically). Digital signals cope very much less well with distortion than Analogue because it has the effect of reducing the noise margin, the minimum Carrier to Noise if you like.

So less distortion can give your signal more DX than more power.

The RF Tech Guy

distortion will always bring issues, hence the need for linearity. Active linearisation is very good indeed, especially with a regular ham radio transceiver.

And from a receive point of view...

The RF Tech Guy wrote: »

To acheive a good BER for a particular modulation scheme, a minimum Carrier to Noise ratio will be required.